Semiconductor lasers in common use today include edge-emitting diode lasers and vertical cavity surface emitting lasers (VCSELs). In an edge-emitting laser, a semiconductor gain medium, for example a quantum-well semiconductor structure, is formed on a surface of a semiconductor substrate. Cavity mirrors are formed or otherwise positioned on opposite sides of the substrate, perpendicular to the substrate surfaces, to form a resonant cavity which includes the gain medium. Electrical or optical pumping of the gain medium generates a laser beam which propagates in a direction along the plane of the substrate.
Edge-emitting lasers are among the most common semiconductor laser devices. Available commercially as individual units and in linear bar arrays, they are used, for example, as an optical pump source for pumping solid state lasers. High power, typically greater than a few hundred milliwatts, adaptations of edge-emitting lasers commonly operate in high order spatial modes and at multiple frequencies. This prevents their use in applications which require high power laser output in a single spatial mode and/or at a single frequency. Edge emitters also have a significant degree of astigmatism and a beam aspect ratio which is generally large, making it difficult to focus the beam to a small spot, which prevents their use in those applications which require a focused output beam. Poor beam quality in edge-mitting lasers also makes frequency doubling of the laser output using nonlinear optical materials difficult and inefficient.
In conventional VCSEL lasers, cavity mirrors are formed or otherwise positioned on opposite faces of a semiconductor gain medium grown on a semiconductor substrate. Electrical or optical pumping generates a laser beam emitted in a direction orthogonal to the plane of the substrate.
Conventional VCSELs find application in optical communications and optical interconnect systems. VCSEL lasers are characterized by generally low fundamental spatial mode TEM.sub.00 output powers, limited to about 8 milliwatts (mW) continuous wave (cw), and are further characterized by small fundamental spatial mode beam diameters, on the order of several micrometers (.mu.m). Larger area VCSEL emitters, with beam diameters on the order of 100 .mu.m can produce output beams having a few hundred mW of cw output power. However, operation of conventional VCSELs at high power and large diameter generally carries with it the penalty of an output beam having high-order spatial modes and multiple frequencies. In an external cavity VCSEL configuration, referred to in the art as a vertical external cavity surface emitting laser (VECSEL), an external reflector serves as the output coupler. External cavity VECSEL devices can provide higher fundamental spatial mode output power than VCSEL devices.
Previous work on external cavity vertically emitting semiconductor lasers typically resulted in low output power. The work of Sandusky and Brueck, for example, produced low output power and used optical pumping to excite the semiconductor. See J. V. Sandusky and S. R. J. Brueck, "A cw external cavity surface-emitting laser", Photonics Technology Letters, vol. 8 pp. 313-315, 1996. In a study by Hadley et al., an electrically excited VCSEL in an external cavity produced 2.4 mW cw and 100 mW pulsed in a fundamental spatial mode. In this case, an emitting area up to 120 .mu.m was used. See M. A. Hadley, G. C. Wilson, K. Y. Lau and J. S. Smith, "High single-traverse mode output from external cavity surface emitting laser diodes", Applied Phys. Letters, vol. 63, pp. 1607-1609, 1993.
For various laser applications, a beam generated by the laser is subjected to frequency conversion or frequency doubling. This is accomplished by introducing a non-linear material, for example KTP, KTN, KNbO.sub.3, and LiNbO.sub.3 into the laser path. The frequency of a beam incident on the non-linear material is converted to a second frequency. The non-linear materials are referred to as "doubling crystals" where the property of the material is such that it serves to double the frequency of a beam traversing the crystal. Efficient frequency conversion by the material generally requires a high-intensity, single mode incident beam.
Frequency doubling of semiconductor lasers has been demonstrated in the past to varying degrees of success using a doubling crystal mounted external to an edge-emitting diode laser cavity. The output beams from edge-mitting diode lasers are usually highly divergent and have significant aspect ratios as well as some degree of astigmatism which degrades the optical field intensity and phase front from that which is ideally required for efficient frequency doubling. Experiments have been carried out in which the light from a diode laser is launched into an optical waveguide fabricated in a non-linear material in order to maintain the optical field intensity over some relatively long path length. This technique is generally complicated and uses relatively low power diode lasers which have sufficient beam quality to launch the laser light into the external waveguide.
Various techniques in the past have attempted to harness beam power to enable efficient conversion. A first technique by Gunter, P. Gunter et al. "Nonlinear optical crystals for optical frequency doubling with laser diodes", Proc. of SPIE, vol. 236, pages 8-18, 1980, demonstrated low efficiency frequency doubling of diode laser radiation using potassium niobate KNbO.sub.3 in a single-pass doubling configuration. In another technique, Koslovsky et al., Optics Letters 12, 1014, 1987, employed a single spatial mode, edge-emitting diode laser and KNbO.sub.3 in an external ring resonator to increase the circulating power to achieve frequency conversion. The Koslovsky configuration required frequency-locking of the single-frequency laser to the Fabry-Perot resonance of the ring cavity as well as matching the temperature of the non-linear crystal to both frequencies. This required complicated crystal alignment and wavelength control circuitry to maintain frequency locking.